Magnetic resonance imaging (MRI) is a medical imaging modality that can create images of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field. When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue or fat become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis. An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z-axis and that varies linearly in amplitude with position along one of the x, y, or z-axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and, in turn, on the resonant frequency of the nuclear spins along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MRI signal by creating a signature resonance frequency at each location in the body. Typically a radio frequency (RF) body coil is used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF body coil is used to add energy to the nuclear spins in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. The RF signal is detected by one or more RF receive coils and is transformed into an image using a computer and known reconstruction algorithms.
The RF receive coil typically includes a large number of individual RF receive elements that may be arranged in a phased array. The size and relative orientations of each of the RF receive elements are tuned to provide the best results for a specific magnetic field strength. Conventional RF receive coils for imaging heads are typically rigid and optimized to fit a patient with a large head. However, in order to obtain the best signal-to-noise ratio, it is important to position the RF receive elements as close to patient's head as possible. Since typical head coils are designed to accommodate a large patient head, there is usually excess room when using a conventional head coil to image a smaller head. This may result in images of reduced image quality. Therefore, for these and other reasons, there is a need for an adjustable MRI head coil that adjusts to fit a wide range of head sizes.